I'd like them to add something like alternate optional Routes and towns/cities away from the main paths, which would have other sidequests and different things to collect, such as Pokemon that only show up in that area, or exclusive outfits (provided customizable trainers actually becomes a feature).

As for the postgame stuff, I'd like there to be a lot of it. Gold and Silver added Kanto, a whole region to explore, almost effectively doubling the gameplay. After those, there hasn't been much in the postgame department, so I'm not expecting much.